The invention relates to the field of gas turbine engines, and more specifically, to gas turbines maintaining fluid density control to control system operation and minimize losses.
In conventional gas turbine engines having a turbine and a compressor, turbine output power is controlled by simply varying the fuel supply. When fuel supply is increased, the temperature upstream of the turbine increases, resulting in increased power and speed. This also causes an increase in pressure and in the expansion ratio. Controlling power in conventional gas turbine engines in this way does not pose any significant problems, but these engines are unable to accommodate sudden load changes because the temperature in the gas turbine engine changes over a very wide range: from 600xc2x0 K. to 1,400xc2x0 K. when operating from idling conditions to full load. In addition, it is not possible to xe2x80x9cscale downxe2x80x9d a conventional gas turbine engine to obtain a lower-power, compact engine for uses such as land vehicle applications because the turbine flow duct fluid parameters would require turbine blades to be as small as xe2x85x9 of an inch in height. With such small blades, the engine would not produce enough torque without requiring a gearbox, which would lower overall efficiency.
These disadvantages can be partly eliminated by reducing the pressure downstream of the turbine with an exhauster. The exhauster allows the expansion ratio to be increased and the pressure upstream of the turbine to be decreased. Turbine blades can then be made larger and consequently produce more torque than otherwise would have been possible. Adding an exhauster does not completely solve the problem, however, because turbine flow duct temperature fluctuations remain. Wide temperature fluctuations cause engine components to incur large thermal expansions and contractions. These deformations result in metal-to-metal clearance fluctuations (which give rise to losses), lower reliability, and reduced service life.
Our co-pending application Ser. No. 09/161,114, filed Sep. 25, 1998 addresses a way to prevent these temperature fluctuations. It discloses a gas turbine engine having a compressor, a power turbine mounted downstream of the compressor, and a compressor turbine for powering the compressor. The compressor turbine is mounted downstream of the power turbine and rotates in a direction opposite to the rotation direction of the power turbine. A heated fluid source is provided upstream of the power turbine and is connected to a fuel source. The engine has a heat exchanger for cooling the waste fluid after the compressor turbine before compressing this waste fluid in the compressor and for heating the waste fluid after the compressor but before feeding this compressed waste fluid to the heated fluid source. To control gas turbine engine power, the fluid density in the engine flow duct is controlled by removing a part of the compressed heated waste fluid leaving the heat exchanger before the compressed waste fluid is fed to the combustor. The removed part of the compressed heated waste fluid is replaced with air for combustion which is fed to the heated fluid source. A pressure booster (compressor) and an auxiliary turbine are used to remove the waste fluid and to replace it with air for combustion.
This approach controls the fluid density in the engine flow duct, thus controlling engine power. The main problem with this density control method is energy loss when the waste fluid is partly removed from the flow duct. In the prior art, the compressed waste fluid is heated in the heat exchanger before a part of the waste fluid is removed from the flow duct of the gas turbine engine. This means that a part of the heat exchanger capacity is used for heating that part of the waste fluid which will then be removed and released into the atmosphere. When this happens, the energy that was used for heating the removed part of the waste fluid is wasted.
Another disadvantage of the prior art is that while the waste fluid-to-air ratio remains stable in the power turbine power range of 50 to 100% when the engine speed remains stable, when the load decreases below 50%, the waste fluid-to-air ratio in the gas mixture going to the combustor changes, with the level of waste fluid becoming higher than needed. As the load decreases, the amount of waste fluid excess increases, especially if the power turbine speed decreases. When the engine goes to the no-load mode, the waste fluid excess can cause flame blowoff. This disadvantage becomes more pronounced when the power turbine speed drops below a certain limit because the fluid velocity leaving the power turbine increases, the losses go up, and the waste fluid outlet temperature of the compressor turbine tends to increase. If the compressor turbine outlet temperature is kept constant by increasing the speed of the compressor, the compressor will produce more waste fluid that will go to the heated fluid source, even as the flow of air fed to the heated fluid source remains the same. This means that the waste fluid excess fed to the heated fluid source will increase, which results in flame blowoff in the heated fluid source. The prior art gas turbine engine consequently cannot work in a stable manner over the entire power range.
It is thus an object of the invention to provide a method of operating a gas turbine engine and a gas turbine engine having a greater efficiency.
Another object of the invention is to provide a method of operating a gas turbine engine in a stable manner over the full power range.
A further object of the invention is to provide a method of operating a gas turbine engine that features a full-range power control system that is simple and effective.
The above objects are accomplished by providing a method of operating a gas turbine engine in which a heated fluid is prepared by burning fuel and combustion air in a mixture with a waste fluid obtained downstream of the compressor turbine, the waste fluid being cooled, compressed and heated before mixing with the fluid and combustion air. The temperature of the waste fluid downstream of the compressor turbine is kept constant. A partial flow of the waste fluid is removed and released into the atmosphere before heating the waste fluid prior to mixing with combustion air.
The gas turbine engine has a fluid compressor, a combustor between the compressor and a power turbine, a counter-rotating compressor turbine installed downstream of the power turbine, and a heat exchanger which has two circuits. One heat exchanger circuit is inserted between the compressor turbine outlet and the inlet compressor, and other circuit is inserted between the compressor turbine outlet and a mixer that has an outlet connected to the combustor and an inlet connected to an air source. The engine has a device to control waste fluid temperature downstream of the compressor turbine, two flow controls to control the mixer inlets, and an outlet that is used to remove waste fluid and release it into the atmosphere between the compressor inlet and outlet.
Other objects and advantages of the invention will become apparent from the following description of preferred embodiments thereof with reference to the accompanying drawings, in which: